1. Field of the Invention
The present invention relates to ferromagnetic nonoparticles which are inexpensive and can be used in an MRAM and a magnetic recording medium, a material coated with a dispersion containing the ferromagnetic nanoparticles, and a magnetic recording medium having a magnetic layer consisting of the material coated with the dispersion containing the ferromagnetic nanoparticles.
2. Description of the Related Art
Magnetic recording media widely used as recording tapes, video tapes, computer tapes, disks, and the like are denser and have shorter recording wavelengths every year. Increasing the S/N ratio in such magnetic recording media is important. The smaller the particle size of ferromagnetic particles, the lower the noise when the total weight of the particles is the same. However, when the particle size of iron particles, which are generally used as ferromagnetic particles, decreases, a phenomenon (superparamagnetism) where magnetization cannot be maintained is caused due to heat fluctuation. Accordingly, there are limits to reducing noise using the conventional iron particles.
In order to overcome this problem, use of ferromagnetic nanoparticles having a particle size of 50 nm or less has been proposed. As for a unitary system, ferromagnetic nanoparticles of Co, Fe, and Ni are described in J. Appl. Phys., Vol. 80, No. 1, pp. 103-108, 1996, and ferromagnetic nanoparticles of Co are described in J. Appl. Phys., Vol. 79, No. 8, Part 2A, pp. 5312-5314, 1996. However, the ferromagnetic nanoparticles have a holding power Hc of only 47.76 kA/m (600 Oe), and there ferromagnetic nanoparticles having a holding power Hc of 95.5 kA/m (1200 Oe), which is required for the magnetic recording media, have not been obtained.
Japanese Patent Application Laid-Open (JP-A) No. 2000-54012 discloses a method for forming ferromagnetic nanoparticles, comprising the steps of: forming a solution of a metal precursor from a transition metal; adding a coagulant to the solution so that nanoparticles are separated from the metal precursor solution without forming permanent agglomeration; and adding a solution of hydrocarbon to the metal precursor solution so that the nanoparticles are recombined or become colloids again.
Further, U.S. Pat. No. 6,162,532 discloses particles containing a magnetic material selected from the group consisting of elements Co, Fe, Ni, Sm, Nd, Pr, Pt, and Gd, intermetallic compounds of the aforementioned elements, binary and ternary alloys of the elements, an Fe oxide further containing at least one of the elements other than Fe, barium ferrite, and strontium ferrite. U.S. Pat. No. 6,162,532 and Science, Vol. 287, 1989 (2000) disclose a method for forming particles by adding a solution of a metal precursor to a surfactant solution.
Science, Vol. 287, 1989 (2000) also discloses a production method of ferromagnetic FePt nanoparticles. A coated product of ferromagnetic nanoparticles is obtained by sedimenting nanoparticles, which are protected only by a surfactant, on the surface of a support in a stationary manner. However, the stationary sedimentation method takes time and is not preferable from an industrial point of view.
Studies in Surface and Catalysis, 132, 243 (2001) suggest synthesis of magnetic nanoparticles in a binder. However, ferromagnetic nanoparticles having a holding power Hc of 95.5 kA/m (1,200 Oe) or more, which are required for the magnetic recording media, cannot be obtained by mere synthesis of magnetic nanoparticles in a binder.